Research will test the hypothesis that free enantiomeric warfarin concerntrations, categorized according to vitamin K concentrations (and/or dietary intake) are the best clinical predictors for warfarin's pharmacologic response in geriatric stroke patients. Aims are: l. Determine, in geriatric stroke patients, pharmacokinetic parameters (area under the curve and oral clearance) for racemic (total and free) and enantiomeric (total and free) warfarin. 2. Measure warfarin's response time in ger. stroke patients using (PT) and INR).